The development of highly absorbent members for use as disposable diapers, adult incontinence pads and briefs, and catamenial products such as sanitary napkins, are the subject of substantial commercial interest. A highly desired characteristic for such products is thinness. For example, thinner diapers are less bulky to wear, fit better under clothing, and are less noticeable. They are also more compact in the package, making the diapers easier for the consumer to carry and store. Compactness in packaging also results in reduced distribution costs for the manufacturer and distributor, including less shelf space required in the store per diaper unit.
The ability to provide thinner absorbent articles such as diapers has been contingent on the ability to develop relatively thin absorbent cores or structures that can acquire and store large quantities of discharged body fluids, in particular urine. In this regard, the use of certain absorbent polymers often referred to as "hydrogels," "superabsorbents" or "hydrocolloid" material has been particularly important. See, for example, U.S. Pat. No. 3,699,103 (Harper et al) issued Jun. 13, 1972, and U.S. Pat. No. 3,670,731 (Harmon) issued Jun. 20, 1972, that disclose the use of such absorbent polymers (hereafter "hydrogel-forming absorbent polymers") in absorbent articles. Indeed, the development of thinner diapers has been the direct consequence of thinner absorbent cores that take advantage of the ability of these hydrogel-forming absorbent polymers to absorb large quantities of discharged body fluids, typically when used in combination with a fibrous matrix. See, for example, U.S. Pat. No. 4,673,402 (Weisman et al) issued Jun. 16, 1987, and U.S. Pat. No. 4,935,022 (Lash et al) issued Jun. 19, 1990, that disclose dual-layer core structures comprising a fibrous matrix and hydrogel-forming absorbent polymers useful in fashioning thin, compact, nonbulky diapers.
Prior to the use of these hydrogel-forming absorbent polymers, it was general practice to form absorbent structures, such as those suitable for use in infant diapers, entirely from wood pulp fluff. Given the relatively low amount of fluid absorbed by wood pulp fluff on a gram of fluid absorbed per gram of wood pulp fluff, it was necessary to employ relatively large quantities of wood pulp fluff, thus necessitating the use of relatively bulky, thick absorbent structures. The introduction of these hydrogel-forming absorbent polymers into such structures has allowed the use of less wood pulp fluff. These hydrogel-forming absorbent polymers are superior to fluff in their ability to absorb large volumes of aqueous body fluids, such as urine (i.e., at least about 15 g/g), thus making smaller, thinner absorbent structures feasible.
Prior absorbent structures have generally comprised relatively low amounts (e.g., less than about 50% by weight) of these hydrogel-forming absorbent polymers. See, for example, U.S. Pat. No. 4,834,735 (Alemany et al) issued May 30, 1989 (preferably from about 9 to about 50% hydrogel-forming absorbent polymer in the fibrous matrix). There are several reasons for this. The hydrogel-forming absorbent polymers employed in prior absorbent structures have generally not had an absorption rate that would allow them to quickly absorb body fluids, especially in "gush" situations. This has necessitated the inclusion of fibers, typically wood pulp fibers, to serve as temporary reservoirs to hold the discharged fluids until absorbed by the hydrogel-forming absorbent polymer.
More importantly, many of the known hydrogel-forming absorbent polymers exhibited gel blocking, especially when included in the absorbent structure at higher levels. "Gel blocking" occurs when particles of the hydrogel-forming absorbent polymer are wetted and the particles swell so as to inhibit fluid transmission to other regions of the absorbent structure. Wetting of these other regions of the absorbent structure therefore takes place via a very slow diffusion process. In practical terms, this means acquisition of fluids by the absorbent structure is much slower than the rate at which fluids are discharged, especially in gush situations. Leakage from the absorbent article can take place well before the particles of hydrogel-forming absorbent polymer in the absorbent structure are fully saturated or before the fluid can diffuse or wick past the "blocking" particles into the rest of the absorbent structure. Gel blocking can be a particularly acute problem if the particles of hydrogel-forming absorbent polymer do not have adequate gel strength and deform or spread under stress once the particles swell with absorbed fluid. See U.S. Pat. No. 4,834,735 (Alemany et al) issued May 30, 1989.
This gel blocking phenomena has typically necessitated the use of a fibrous matrix in which are dispersed the particles of hydrogel-forming absorbent polymer. This fibrous matrix keeps the particles of hydrogel-forming absorbent polymer separated from one another. This fibrous matrix also provides a capillary structure that allows fluid to reach the hydrogel-forming absorbent polymer located in regions remote from the initial fluid discharge point. See U.S. Pat. No. 4,834,735 (Alemany et al) issued May 30, 1989. However, dispersing the hydrogel-forming absorbent polymer in a fibrous matrix at relatively low concentrations in order to minimize or avoid gel blocking can lower the overall fluid storage capacity of thinner absorbent structures. Using lower concentrations of these hydrogel-forming absorbent polymers limits somewhat the real advantage of these materials, namely their ability to absorb and retain large quantities of body fluids per given volume. In addition, the particles of hydrogel-forming absorbent polymer may not be immobilized in this fibrous matrix and may be free to migrate during processing and/or use. This migration of particles, especially as the hydrogel-forming absorbent polymer swells, can contribute to gel blocking.
One method for increasing the relative concentration of hydrogel-forming absorbent polymer is to form a layer thereof between two other fibrous layers, e.g., a laminate structure. See, for example, U.S. Pat. No. 4,600,458 (Kramer et al) issued Jul. 15, 1986, and U.S. Pat. No. 5,009,650 (Bernardin) issued Apr. 23, 1991. The fibrous layers of prior laminate structures have often been held together by hydrogen bonding as a result of spraying the fibrous layers with water, typically followed by compaction. See U.S. Pat. No. 4,260,443 (Lindsay et al) issued Apr. 7, 1981, U.S. Pat. No. 4,360,021 (Stima) issued Nov. 23, 1982, and U.S. Pat. No. 4,851,069 (Packard et al) issued Jul. 25, 1989. These prior laminate structures have also been held together by adhesive bonding between the fibrous layers or between the fibrous layers and the hydrogel-forming absorbent polymer. See U.S. Pat. No. 4,994,053 (Lang) issued Feb. 19, 1991 (pattern of adhesive or heat fusible films), and U.S. Pat. No. 5,128,082 (Makoui) issued Jul. 7, 1992 (latex coating). By encapsulating the particles of hydrogel-forming absorbent polymer between these fibrous layers (especially those held together by hydrogen bonding or adhesive bonding), the overall particle mobility within the absorbent structure is also greatly reduced.
As the hydrogel-forming absorbent polymer is contacted with fluid, it can swell to a significant volume. This can cause problems with laminate structures, especially those containing high concentrations of hydrogel-forming absorbent polymer. The fibrous layers of the laminate can restrict the ability of the hydrogel-forming absorbent polymer to expand as it is contacted with additional fluid, thus limiting the overall fluid capacity of the structure. Laminates held together by hydrogen bonding can have insufficient integrity, especially as the structure becomes saturated with fluid. Adhesive bonding, especially when the adhesive is hydrophobic and/or coats the hydrogel-forming absorbent polymer, can also decrease the total fluid capacity of the structure.
Some of these prior laminate structures have involved the formation of discrete, spaced pockets of hydrogel-forming absorbent polymer particles. See U.S. Pat. No. 4,360,021 (Stima) issued Nov. 23, 1982 (backsheet 13 and cover sheet 15 made of tissue wadding that is selectively compressed to form pockets of hydrogel-forming absorbent polymer particles 24), U.S. Pat. No. 4,994,053 (Lang) issued Feb. 19, 1991 (pattern of adhesive 60, as well as heat fusible films 68 and 70, to form pockets of hydrogel-forming absorbent polymer particles 56/58), U.S. Pat. No. 4,055,180 (Karami) issued Oct. 25, 1977 (retaining sheet 56 that is preferably a thermoplastic film fused to wadding sheet 40 by heat to form pockets of hydrogel-forming absorbent polymer particles 62), and U.S. Pat. No. 5,149,335 (Kellenberger et al) issued Sep. 22, 1992 (outer cover 22 bonded to body-side liner 24 along bond lines 28 to form compartments 30 of hydrogel-forming absorbent polymer particles 32).
Laminate structures having these discrete pockets of particles form an essentially "discontinuous" layer or pattern of hydrogel-forming absorbent polymer. Since the hydrogel-forming absorbent polymer is a discontinuous layer, it may be more difficult for the fluid to reach each of the discrete pockets. Indeed, in some of these "discontinuous" laminate structures, the discrete pockets of hydrogel-forming absorbent polymer are not in direct fluid communication. Moreover, these discrete pockets of hydrogel-forming absorbent polymer can form protuberances that are undesirable aesthetically and can make the absorbent article uncomfortable for the wearer.
Laminate structures based on discrete pockets of hydrogel-forming absorbent polymer particles also do not take much advantage of any inherent gel permeability of the hydrogel formed when these absorbent polymers swell in the presence of body fluids. It is believed that when a hydrogel-forming absorbent polymer is present at high concentrations in an absorbent structure and then swells to form a hydrogel under usage pressures, the boundaries of the hydrogel come into contact, and interstitial voids in this high-concentration region become generally bounded by hydrogel. When this occurs, it is believed the gel permeability properties of this region are generally reflective of the permeability properties of a hydrogel zone or layer formed from the hydrogel-forming absorbent polymer alone. This allows this hydrogel layer to transport and distribute fluids at rates that more closely approach those of a fibrous web. Hydrogel layers that are formed as discrete, discontinuous pockets will only marginally utilize, if at all, these inherent gel permeability properties.
Accordingly, it would be desirable to provide absorbent structures that: (1) have high concentrations of hydrogel-forming absorbent polymer; (2) can immobilize the hydrogel-forming absorbent polymer without restricting the ability of it to expand as it is contacted with additional fluid; and (3) takes advantage of the inherent gel permeability properties of the resulting hydrogel that forms.